1. Field of the Invention
This invention relates to integrated-circuit (IC) chips having semiconductive switches such as might be used in multiplexers to connect any one of a number of inputs to an output. More particularly, this invention relates to so-called fault-protected overvoltage switches.
2. Description of the Prior Art
Fault-protected overvoltage switches have been provided commercially for some time. Such switches typically are arranged to open automatically if an excessive voltage is applied to the switch input (or the output in some cases), thereby preventing the overvoltage from reaching and damaging a device to which the switch is connected. Commonly, such switches comprise a number of series-connected MOSFET transistors. For example, they may include an N-channel MOSFET, a P-channel MOSFET, and another N-channel MOSFET, in that order.
One such three-transistor circuit is described in copending application Ser. No. 08/227,727, filed by the present inventors on Apr. 14, 1994. In that device, each transistor is placed in a corresponding tub of the IC chip. Each of these tubs is electrically isolated from all other sections of the IC chip, so that the MOS transistors are isolated from one another and from the chip voltage supplies. Although that device performs satisfactorily in most respects, its operating voltage range is somewhat less than desired due to body effect in the P-channel transistor.